This specification relates to challenge-response tests.
Challenge-response tests aim to distinguish real users, i.e., humans, from computer programs. Some challenge-response tests that exist show a user a distorted image of text and ask the user to enter the text in the image. The image of the text is selected so that it would be difficult for a computer program to automatically correctly decipher the text correctly, e.g., using an Optical Character Recognition (OCR) technique. Some challenge-response tests provide the user with images where the correct text in the image is unknown to the providing system, e.g., along with an image where the correct text in the image is known to the system. The system can use user responses that identify the text in the image to determine how to correctly decipher the image text.